In a technical field such as optical communication, integrated optical components utilizing an optical waveguide structure have been developed to construct optical circuits for easily implementing optical splitting and interference. The integrated optical components utilizing the property of waves make it to adjust optical path length and to facilitate fabrication of interferometers. In addition, applying the semiconductor circuit processing technique enables integration of optical components.
However, since these optical waveguide circuits construct individual components of optical circuits using an “optical confinement structure” that carries out the spatial optical confinement of light propagating through an optical waveguide by utilizing the spatial distribution of refractive index, a cascaded circuit design using optical wiring is required. Consequently, it is inevitable that optical path length of the optical waveguide circuit becomes longer than the optical path length required for bringing about interference in the optical circuit. This presents a problem of sharply increasing the size of the optical circuit.
For example, take a typical arrayed waveguide grating as an example. The light including a plurality of wavelengths (λj) input to an input port repeats demultiplexing/multiplexing through star couplers using slab waveguides, and demultiplexed optical waves are output from an output port. In this case, an optical path length required to demultiplex the optical waves at a resolution of an order of one thousandth of the wavelength becomes several tens of thousands of the wavelength of the light propagating through the waveguide. Furthermore, it is necessary not only to carry out waveguide patterning of the optical circuit, but also to perform such processing as providing a wave plate for correcting the circuit characteristics that depend on the polarization state (see, for example, Y. Hibino, “Passive optical devices for photonic networks”, IEIC Trans. Commun., Vol. E83-B No. 10, (2000)).
In addition, since it is necessary to closely confine the light within the waveguide to miniaturize the optical circuit, the optical waveguide must have a very large refractive index difference to control the optical confinement state by the spatial distribution of the refractive index. For example, a conventional step-index optical waveguide is designed such that it has the spatial distribution of the refractive index that will make the refractive index difference greater than 0.1%. The optical confinement utilizing such a large refractive index difference presents a problem of reducing the flexibility of a circuit configuration. In particular, when implementing the refractive index difference in optical waveguide by local ultraviolet irradiation, thermooptic effect or electrooptic effect, the amount of change in the resultant refractive index is about 0.1% at best. Accordingly, to change the propagation direction of light, it must be gradually varied along the optical waveguide. Thus, the optical circuit inevitably becomes long, which makes it difficult to miniaturize the optical circuit.
Furthermore, an optical circuit including a grating-like circuit in addition to an optical waveguide circuit is built on a basis of a periodic structure that is substantially parallel to the propagation direction of light, or of the periodic variation in a dielectric refractive index. In addition, in an actual design, the characteristics of the optical circuit are usually achieved by a strictly periodic structure evaluable by Fourier transform, or by a chirped structure that distorts the periodicity slightly. Consequently, the optical circuit has a substantially uniform structure for a wavefront, thereby making it difficult to control the light in the direction perpendicular to the propagation direction (in the direction of the wavefront). For example, an optical circuit disclosed in T. W. Mossberg, “Planar holographic optical processing”, Optics Letters, Vol. 26, No. 7, pp. 414-416 (2001) cannot utilize the light that propagates through the optical circuit without reflection as signal light because it spreads in the circuit. In addition, as for a circuit that varies a spot position sharply in a direction perpendicular to the propagation direction, such as a branching circuit, since the optical field must be greatly expanded in the direction perpendicular to the propagation direction, the size of the circuit inevitably becomes large. Furthermore, in the actual circuit design, only a design method is applicable which is nearly equivalent to the design method of a conventional linear grating circuit such as a fiber grating. Thus, the design is limited to a circuit with a strictly periodic structure (that is, an optical circuit depending on the wave number in the propagation direction). This increases the circuit scale, makes the circuit sensitive to the wavelength, and has the  input/output positions distributed sequentially in accordance with the wavelengths, thereby presenting a problem of limiting the design to circuits with little flexibility.
On the other hand, optical wavelength division multiplexing communication systems using a plurality of optical wavelengths have been developed actively to increase communication capacity. Such an optical wavelength division multiplexing communication system employs an arrayed waveguide grating type optical multi/demultiplexing circuit as an optical wavelength multi/demultiplexing circuit for multiplexing optical signals with a plurality of wavelengths at a transmitter side, and for demultiplexing the plurality of optical signals propagating through an optical fiber to different ports at a receiver side.
FIG. 1 is a diagram illustrating a configuration of a conventional arrayed waveguide grating type optical multi/demultiplexing circuit (see, for example, K. Okamoto, “Fundamentals of Optical Waveguides”, Academic Press (2000)). The circuit comprises on a substrate 100 an input waveguide 101, a first slab waveguide 102, arrayed waveguides 103, a second slab waveguide 104 and output waveguides 105, which are connected in this order.
The light launched into the input waveguide 101 is expanded by the first slab waveguide 102, and is demultiplexed to the arrayed waveguides 103 composed of waveguides configured based on individual wavelengths. Then, the outputs of the arrayed waveguides 103 are multiplexed by the second slab waveguide 104 again to be led to the output waveguides 105. Here, the optical field pattern projected to the end of the first slab waveguide 102 on the side of the arrayed waveguides 103 is basically reproduced (copied) at the end of the second slab waveguide 104 on the side of the arrayed waveguides 103. However, since the arrayed waveguides 103 are designed such that the optical path lengths of their adjacent optical waveguides differ by ΔL, the optical field has an inclination depending on the wavelength of the input light. The inclination causes the positions of the focuses the optical field forms on the end of the second slab waveguide 104 on the side of the output waveguides 105 to be changed for the respective wavelengths, thereby enabling the wavelength demultiplexing.
Such an arrayed waveguide grating type optical multi/demultiplexing circuit has become an indispensable optical component for an optical multiplexing communication system that transmits an optical signal with a plurality of wavelengths through a single optical fiber. In addition, a variety of extended-passband arrayed waveguide grating type optical multi/demultiplexing circuits have been proposed which increase the transmission wavelength bandwidth of the arrayed waveguide grating type optical multi/demultiplexing circuit as shown in FIG. 1.
FIGS. 2A and 2B are diagrams illustrating a configuration of a conventionally proposed extended-passband arrayed waveguide grating type optical multi/demultiplexing circuit (see, for example, K. Okamoto and A. Sugita, “Flat spectral response arrayed-waveguide grating multiplexer with parabolic waveguide horns”, Electronics Letters, Vol. 32, No. 18, pp. 1661-1662 (1996)).
As shown in FIG. 2A, the circuit is configured by adding to the circuit as shown in FIG. 1 a parabolic waveguide 106 as illustrated in FIG. 2B, which is placed between the input waveguide 101 and the first slab waveguide 102. In FIG. 2B, z denotes the propagation direction of light.
FIGS. 3A and 3B are diagrams illustrating optical field distribution at an interface between the parabolic waveguide 106 and slab waveguide 102a in the configuration of FIG. 2A, in which z denotes the propagation direction of light, and x denotes the direction of a cross section of the waveguide perpendicular to the z direction. As shown in FIG. 3B, the optical field distribution has a double-peak profile. The double-peak optical field is regenerated at the output waveguide side of the second slab waveguide 104, and is coupled to the output waveguides 105, thereby implementing the extended transmission wavelength band.
The extended-passband arrayed waveguide grating type optical multi/demultiplexing circuit with the above-mentioned conventional configuration, however, has a large chromatic dispersion value due to the phase distribution in the parabolic waveguide as illustrated in FIG. 4. Since the chromatic dispersion provides different delay times to signal spectral components, the conventional extended-passband arrayed waveguide grating type optical multi/demultiplexing circuit with the large chromatic dispersion has a problem of bringing about considerable optical pulse degradation.